490 GENERAL CONSIDERATIONS 



and which is known as an antibody, or antitoxin. Such a substance is 

 produced by the body in response to the presence of any foreign chemical 

 substance and is part of the self-regulatory function of the body by 

 which it can adjust or defend itself. By neutralizing the toxins, the 

 antitoxins safeguard the body cells against injury and give time for the 

 body to eliminate the cause of the disturbance. Since the response to 

 each toxin is specific, a different antitoxin is produced for each 

 one. 



549. How the Body Fights Disease. — One method by which the body 

 fights disease is, as has already been indicated, by the production of 

 antitoxins. Another way is through the activity of the white blood 

 corpuscles, or leucocytes. A leucocyte is an ameboid cell which shows 

 a tendency to take into its body other organisms and other materials 

 in the same fashion as an ameba takes in bits of food. Normal body 

 cells are not attacked by the leucocytes, but cells in the body which 

 become abnormal or which are injured, or foreign cells of any kind, 

 are taken up by them and destroyed. When thus taking in other cells 

 they are termed -phagocytes (literally, eaters of cells). Phagocytes are 

 active in the destruction of certain cells in the body when the absorption 

 of tissue is desirable. For example, they play a part in the absorption 

 of the tail of a tadpole when it changes into a frog. When injury results 

 in the death and destruction of cells in the body, the phagocytes attack 

 the dead and injured cells and by destroying them and clearing away 

 cellular debris pave the way for normal regeneration and the return of a 

 healthful condition. They are also active whenever disease-producing 

 organisms enter the body. Attracted to the place of entrance of these 

 infective organisms, apparently in response to the unusual chemical 

 stimuli due to the invaders, the phagocytes ingest and destroy them. 

 If the number of invading organisms is not great, the phagocytes may in 

 this way safeguard the body against the onset of disease. If, however, 

 the invading organisms are so numerous at the point of infection that the 

 leucocytes are unable to cope with them, then enough tissue may be 

 broken down to cause the formation of a pus cavity, or abscess, and the 

 leucocytes become -pus cells. 



550. Immunity. — Immunity may be defined briefly as the absence 

 of susceptibility to disease. It may be of three kinds: natural, acquired, 

 and artificial. Acquired immunity may also be inherited. 



Natural immunity is possessed by an animal because of the character 

 of its body. Many animals are naturally immune to certain diseases 

 to which others are susceptible. More or less immunity to some diseases 

 is possessed by certain human races; for example, the Jewish race is very 

 resistant to tuberculosis, while Negroes and the Irish are particularly 

 susceptible to it. There is also age immunity, adults being generally 

 free from so-called children's diseases. 



